1. Field of the Invention
This invention relates to a disk array device and a method of extending the storage capacity of the disk array device. More particularly, it relates to a disk array device adapted to extend the storage capacity thereof by adding disks, while maintaining the RAID type of the disk array and a method of extending the storage capacity of such a disk array device.
2. Related Background Art
A disk array is an arrangement of a plurality of disk devices in order to enhance the performance and the reliability of a disk system and, at the same time, realize a large storage capacity at reduced cost. Six categories of RAID (redundant array of inexpensive disks) 1 through 6 are known, although each of them have advantages and disadvantages so that they are selectively used depending on applications. For instance, RAID5 is designed to distribute parities to a plurality of disks on a stripe by stripe basis in order to avoid the disadvantage of RAID4 that the redundancy disks for parities is a bottle neck. It is conveniently used for transaction processing systems. However, RAID5 has several types that are defined by the number of disk devices that constitute a redundancy group. In this letter of specification, each type of RAID 5 is indicated by annexing (iD+P) to RAID5 as conventional expression of the technological field of disk arrays, where i represents a positive integer not smaller than 2 and D represents a data disk, while P represents a parity disk. However, it should be noted that parities are distributed among a plurality of disk devices for RAID5 as described above so that there is no disk device dedicated to parities.
While one of the advantages of a disk array is its large storage capacity, the volume of data to be stored is ever increasing so that the storage capacity of a disk array being operated may have to be extended from time to time. Three techniques as listed below are known to extend the storage capacity of a disk array of RAID5 by adding a disk device.
Known Technique (1)
All the data stored in the disk array of RAID5 that is being operated are temporarily saved in a magnetic tape or the like and the RAID configuration of the disk array is altered so as to match the added disk device before all the saved data are restored. For example, when a disk device is added to a disk array of RAID5 (3D+1P) comprising four disk devices, all the data in the disk array of RAID5 (3D+1P) are temporarily saved in a magnetic tape and the disk array is reconfigured to realize RAID5 (4D+1P). Then, all the saved data are restored.
Known Technique (2)
The parity group is reconfigured and the parities are recomputed while the existing data stored in the disk array of RAID5 that are being operated are moved to the added disk device (see, JP 8-115173 A). For example, when a disk device is added to a disk array of RAID5 (3D+1P), the disk array is reconfigured to realize RAID5 (4D+1P). For the feature of extending RAID5 (3D+P) to RAID5 (4D+P), see “RAID Subsystem G Series (Desktop Model, 19″ Rack Model) Handling Manual”, P/NA207750, Revision 4.4. A summary of the extension feature is also described on the Internet at <URL:
http://www.adtx.com/product/manuals/subsystem/axrs-g/l_rsr-gxs_usr44—7750.pdf>, page 13.
Known Technique (3)
This technique is an improvement to the known technique (2) and designed to eliminate the task of recalculating the parities of RAID by incorporating the added disk device after writing nil data in the entire storage region of the added disk device (see, JP 2000-10738 A).
While several techniques have been put to use to extend the storage capacity by adding a disk device to the disk array being operated, the RAID configuration of the disk array is altered each time a disk device is added by using any of the known techniques. For instance, when the initial RAID configuration is RAID5 (3D+P), it is altered to RAID5 (4D+P) when a disk device is added and to RAID5 (5D+P) when two disk devices are added with any of the known techniques (1) through (3).
Generally speaking, the reliability of RAID5 (3D+P), that of RAID5 (4D+P) and that of RAID5 (5D+P) are equal from the viewpoint of capability of degeneration to a single unit. However, the probability of forced degenerative operation is higher at RAID5 (4D+P) than at RAID5 (3D+P) and higher at RAID5 (5D+P) than at RAID5 (4D+P). Differently stated, the larger the value of i in RAID5 (iD+P), the higher the probability of forced degenerative operation. This is because the probability of producing a faulty disk device increases as the number of disk devices that constitute a redundancy group increases. Additionally, as the value of i increases, the number of disk devices that are to be accessed in parallel during a degenerative operation increases to raise the processing overhead. On the other hand, the efficiency of utilization of data falls as the value of i decreases. For this reason, an optimum value is selected for i by referring to the designed applications when a disk array is prepared initially. However, there arises a problem that the initial configuration of RAID cannot be maintained when the storage capacity of a disk array is extended by means any of the known techniques.
Additionally, with the known technique (3), the parities are not distributed among all the disks at the time of adding a disk device so that a perfect RAID5 configuration is not realized to consequently reduce the data protection capability of the disk array.